There is an urgent need for orderly development of additional sources of hydrocarbon if the national goal of energy independence is to be reached. One of the possible sources for at least a portion of our energy needs is tar sands, which are a consolidated mixture of bitumen (tar) and sand. The bitumen consists of a mixture of a variety of hydrocarbons and, if properly separated from the sand component, can be used as a feedstock for the production of synthetic fuels and/or petrochemicals. For example, the tar sand deposits of the intermountain region of the western United States have an estimated reserve of more than twenty-eight billion barrels of oil in place. Although this resource is only a small fraction of the total U.S. oil requirement, it could be an important source of hydrocarbons on a regional basis.
The nature of tar sands varies greatly depending upon their geographical source insofar as certain tar sand deposits are more easily processed than others. For example, Athabasca tar sands from Alberta, Canada, have an average bitumen content of 12-13 weight percent and a relatively high moisture content of about 3-5 weight percent. It is believed that these tar sands consist of aggregates of sand, wherein each grain of sand is surrounded by a film of connate water, which separates the bitumen from the sand grains. This structure permits easier separation on a larger scale. In fact, commercial hot water extraction processes for recovering bitumen from Athabasca tar sands presently exist. A good review of the Alberta tar sands projects is presented in an article entitled "Tar Sands: A New Fuels Industry Takes Shape," Science, Vol. 199, page 756 (February 1978).
On the other hand, those deposits of tar sands found in the intermountain region of the United States have an average of less than about 10 weight percent bitumen and negligible amounts of connate water, hence the bitumen is in direct contact with the grains of sand. This situation makes recovery much more difficult. Examples of such tar sands include the Sunnyside and Asphalt Ridge deposits found in Utah. A further type of tar sand found mostly in California is the diatomaceous earth type, which contains up to about 25 weight percent bitumen. In this type of deposit the bitumen is contained within the very fine pores of the matrix and consequently is very difficult to extract. It also yields a large amount of fines upon extraction.
In addition to a variety of aqueous extraction processes as just mentioned, other extraction processes have been disclosed which, among other features, use different solvents. For example, U.S. Pat. No. 3,941,679 discloses the use of trichlorofluoromethane as an extraction solvent. U.S. Pat. No. 4,036,732 discloses the use of paraffinic hydrocarbons having from 5 to 9 carbon atoms. U.S. Pat. No. 4,046,663 teaches the use of a naphtha/methanol solvent system.
In selecting a solvent system for tar sands extraction, a number of factors must be considered in evaluating performance. Of course the most obvious factor is the effectiveness of the solvent in separating the bitumen from the sand. This is often counterbalanced by a second consideration, however, which is the asphaltene content of the recovered bitumen. Asphaltenes are complex high molecular weight materials which are undesirable for subsequent refining processes. In this regard, an article entitled, "The Solubility of Asphaltenes in Hydrocarbon Solvents," by D. L. Mitchell and J. G. Speight, Fuel, Vol. 52, pp. 149-152 (1973), extensively explores the solubility of asphaltenes for over fifty different solvents and blends.
A third factor of importance is the fines or mineral particle content of the extracted bitumen. Fines initially present in the tar sands as well as fines formed during grinding operations in preparation for extraction pose difficult separation problems. The rate at which the fines settle is to a large extent dependent upon the solvent used and has been thought to be primarily determined by the density and viscosity of the solvent.